Downy Mildew of Onion Symptoms and Treatment

What does downy mildew look like? What is the causative agent of downy mildew? How to control downy mildew? First, learn about the downy mildew of onion symptoms and treatment guidelines!

Downy Mildew of Onion Definition

Downy mildew of onion means – What is downy mildew of onion?

Downy mildew is a disease that hurts onions and makes their leaves and bulbs grow less well and produce less. Epidemics can only start when it’s cool and rainy for a long time. This means that severe attacks are rare.

Downy mildew has become more of a problem in the UK in recent years, especially since the number of overwintering bulb onions has increased. However, it is hard to control the disease with fungicides, and finding better ways to deal with diseases is a top research priority.

downy mildew of onion symptoms and treatment - downy mildew of onion chemical control

Downy Mildew of Onion Causes

What causes downy mildew of onion? What is the most common cause of the downy mildew of onion?

Downy mildew is caused by an oomycete called Peronospora destructor. This obligate pathogen makes sporangia with thin walls that measure 40–72 x 18–29 μm.

Sporangiophores come out through stomata when the temperature is between 4–25º C. Globose oospores, which are 30–44 μm in diameter, are found in onion bulbs, crop residues, and volunteer plants.

Peronospora destructor only attacks alliums. It affects many wild and cultivated alliums, like onion, garlic, leek, chives, and shallot. Allium cepa is, however, the most severely affected host.

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Downy Mildew of Onion Symptoms

Signs and symptoms of downy mildew of onion – What are some symptoms of downy mildew of onion?

The first signs of downy mildew are yellowing of the tips of the leaves and small, irregularly shaped spots of chlorosis on the leaves. As the disease worsens, the chlorotic spots get bigger and join together to form large lesions up to 10–15 cm long.

As time goes on, these spots that are sick turn brown. Purple, fuzzy sporula­tion grows on the affected spots, and this kind of growth often forms in concentric zones. The leaf can get lesions that make it bend over and fall over. There are also lesions on the flower stalks, which can get girdled and fall over.

The whole crop can lose all of its leaves in just four disease cycles when there are big epidemics. If downy mildew is bad while the bulbs grow, they may not produce as many.

Lesions can be taken over by Stemphylium fungi, which make dark brown to black sporulation and sometimes purple pigmentation inside the downy mildew lesion. Downy mildew can spread through a plant’s entire body, and infected bulbs get soft and change colour when stored; some even start to grow too soon.

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Downy Mildew of Onion Disease Cycle

Infection requires cool temperatures (less than 22º C) and free moisture on the leaf for at least 3 hours. The best temperature for sporangia to sprout is 10–12º C. Germ tubes form in 2 to 4 hours, produce an appressorium, and enter the leaf through stomata.

Spores are made at night and spread by the wind during the day. They can live on leaf surfaces for up to 3 days. When the weather is dry, diseases can’t grow. The pathogen overwinters as oospores and mycelium in the bulbs, and oospores can also live in the soil.

Downy mildew epidemics seem to be less bad the following year when the winter is colder. This is likely because fewer of the pathogens survive. Seeds can spread the pathogen, but this is not considered important.

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Downy Mildew of Onion Treatment Guidelines

Treatment of downy mildew of onion – What is the best treatment for downy mildew?

I hope you understand the downy mildew of onion symptoms and treatment guidelines.

Plant disease-free onion sets and transplants. If you heat infected sets, the pathogen inside them might not be able to live as long. There are differences in how susceptible different cultivars are, so plant cultivars and Allium species are less likely to get sick.

Changing the order of the onion crops. This strategy does not eliminate airborne sporangia, but it may help reduce the number of spores in the soil after 3 to 4 years. Use crop leftovers as soon as possible after harvesting. Control volunteer plants. Choose fields that get a lot of airflows. Sheltered places should be avoided because they are good places for downy mildew to grow.

Crop rows aligned with the wind’s direction will also help dry out the area around the crops.

Fungicides should be used early on before a disease becomes serious. Unfortunately, protectants like dithiocarbamates and copper can’t heal lesions already there, so the systemic drug mefenoxam is often used along with the protectants.

Disease prediction or decision-making systems like DOWNCAST and ZWIPERO are being tested in the field.

I hope you understand the downy mildew of onion symptoms and treatment guidelines.

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